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Chemistry LibreTexts

5.6: Redox Reactions in Organic Chemistry and Biochemistry

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  • Page ID
    68001
  • Skills to Develop

    • To identify oxidation-reduction reactions with organic compounds.

    Oxidation-reduction reactions are of central importance in organic chemistry and biochemistry. The burning of fuels that provides the energy to maintain our civilization and the metabolism of foods that furnish the energy that keeps us alive both involve redox reactions.

    Figure \(\PageIndex{1}\): The Burning of Natural Gas. The burning of natural gas is not only a combustion reaction but also a redox reaction. Similar reactions include the burning of gasoline and coal. These are also redox reactions. Image used with permission from Wikipedia.

    All combustion reactions are also redox reactions. A typical combustion reaction is the burning of methane, the principal component of natural gas (Figure \(\PageIndex{1}\)).

    \[CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O \label{Eq1}\]

    In respiration, the biochemical process by which the oxygen we inhale in air oxidizes foodstuffs to carbon dioxide and water, redox reactions provide energy to living cells. A typical respiratory reaction is the oxidation of glucose (C6H12O6), the simple sugar we encountered in the chapter-opening essay that makes up the diet of yeast:

    \[C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O \label{Eq2}\]

    In food chemistry, the substances known as antioxidants are reducing agents. Ascorbic acid (vitamin C; C6H8O6) is thought to retard potentially damaging oxidation of living cells. In the process, it is oxidized to dehydroascorbic acid (C6H6O6). In the stomach, ascorbic acid reduces the nitrite ion (NO2) to nitric oxide (NO):

    \[C_6H_8O_6 + 2H^+ + 2NO_2^− \rightarrow C_6H_6O_6 + 2H_2O + 2NO \label{Eq7}\]

    If reaction in Equation \(\ref{Eq7}\) did not occur, nitrite ions from foods would oxidize the iron in hemoglobin, destroying its ability to carry oxygen.

    Tocopherol (vitamin E) is also an antioxidant. In the body, vitamin E is thought to act by scavenging harmful by-products of metabolism, such as the highly reactive molecular fragments called free radicals. In foods, vitamin E acts to prevent fats from being oxidized and thus becoming rancid. Vitamin C is also a good antioxidant (Figure \(\PageIndex{2}\)).

    Figure \(\PageIndex{2}\): Citrus Fruits. Citrus fruits, such as oranges, lemons, and limes, are good sources of vitamin C, which is an antioxidant. Wedges of pink grapefruit, lime, and lemon, and a half orange (clockwise from top). Image used with permission from Wikipedia.

    Finally, and of greatest importance, green plants carry out the redox reaction that makes possible almost all life on Earth. They do this through a process called photosynthesis, in which carbon dioxide and water are converted to glucose (C6H12O6). The synthesis of glucose requires a variety of proteins called enzymes and a green pigment called chlorophyll that converts sunlight into chemical energy (Figure \(\PageIndex{3}\)). The overall change that occurs is as follows:

    \[6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2 \label{Eq4}\]

    In this reaction, carbon dioxide is reduced to glucose, and water is oxidized to oxygen gas. Other reactions convert the glucose to more complex carbohydrates, plant proteins, and oils.

    Figure \(\PageIndex{3}\): Life on Earth. Photosynthesis is the fundamental process by which plants use sunlight to convert carbon dioxide and water into glucose and oxygen. Then plants make more complex carbohydrates. It is the ultimate source of all food on Earth, and it is a redox reaction. Image used with permission from Wikipedia.

    Summary

    Redox reactions are common in organic and biological chemistry, including the combustion of organic chemicals, respiration, and photosynthesis.

    Concept Review Exercise

    1. Give some biochemical examples of oxidation and reduction reactions.

    Answer

    1. photosynthesis and antioxidants in foods (answers will vary)

    Exercises

    1. A typical respiratory reaction discussed in the text is the oxidation of glucose (C6H12O6):

      C6H12O6 + 6O2 → 6CO2 + 6H2O

      Is this a redox reaction? If so, what are the oxidizing and reducing agents?

    2. The major net reaction in photosynthesis is as follows:

      6CO2 + 6H2O → C6H12O6 + 6O2

      Is this a redox reaction? If so, what are the oxidizing and reducing agents?

    3. What would be the ultimate organic product if CH3CH2CH2OH were to react with a solution of K2Cr2O7?

    4. What would be the ultimate organic product if CH3CH2CH2OH were to react with a solution of K2Cr2O7?

    5. What would be the final organic product if CH3CH2CHOHCH3 were to react with a solution of K2Cr2O7?

    6. What would be the major organic product if CH3CH2CHOHCH2CH3 were to react with a solution of K2Cr2O7?

    7. What alcohol is produced in the reduction of acetone [(CH3)2CO]?

    8. What alcohol is produced in the reduction of propanal (CH3CH2CHO)?

    Answers

    1. yes; oxidizing agent: O2; reducing agent: C6H12O6

     
    1. CH3CH2COOH

     
    1. CH3CH2C(O)CH3, where the carbon is double bonded to the oxygen

     
    1. CH3CHOHCH3, or isopropyl alcohol